A Rankine circuit constructing a system for utilizing waste heat of an inner combustion engine, such as a vehicle engine, has a circulation path through which a working fluid (heat medium) circulates. A pump, an evaporator (heat exchanger), an expansion machine, and a condenser are interposed in the circulation path in the order named.
The pump is driven, for example, by an electric motor to circulate the working fluid. The working fluid receives waste heat when passing through the evaporator, and is expanded in the expansion machine. In this process, the thermal energy of the working fluid is converted into torque and outputted outside. The thermal energy is thus used, for example, to rotate a fan for air-cooling the condenser.
As a compact and inexpensive fluid machine that is suitable for the Rankine circuit, Unexamined Japanese Patent Publication No. 2005-30386 discloses a fluid machine in which a pump, an expansion machine and a motor have a single drive shaft in common. In the fluid machine, the pump is started by the motor being activated upon receipt of external power. The working fluid circulates in response to the start of the pump, and the working fluid that has received heat energy is expanded in the expansion machine. After the motor is activated, the power supply to the motor is stopped. The pump is operated by the torque outputted from the expansion machine, and the motor is caused to function as a generator.
According to the fluid machine disclosed in Publication No. 2005-30386, since the motor that produces a rotational driving force is equipped with a function of generating electric power, the power generation efficiency of the motor is lower than that of a generator having only the power-generating function.
The fluid machine disclosed in Publication No. 2005-30386 utilizes a DC motor. In general, a DC motor has larger weight than an AC motor, and has lower power generation efficiency when being used as a generator. In addition, the DC motor needs to be maintained by brushing.
Furthermore, according to the fluid machine described in Publication No. 2005-30386, the heat energy recovered by the expansion machine, that is, the torque produced in the expansion machine, is once converted into electric power. The recovered heat energy cannot be outputted to the outside as torque.